Surface-mount connectors such as BGA connectors typically include a plurality of electrically-conductive contacts mounted in a housing. Each conductor has a solder ball attached to a tail portion thereof. The solder balls collectively form a ball grid array.
The solder balls are used to form electrical and mechanical connections between the connector and a substrate, such as a printed-circuit board (PCB), on which the connector is mounted. The connector is mounted on the substrate by heating the solder balls to their melting point. The molten solder subsequently cools and re-hardens to form solder connections between the connector and the substrate.
The solder balls can be heated by placing the connector and the substrate in a convection reflow oven. The oven directs heated air over the connector. Heat is transferred to the solder balls directly and indirectly, by a combination of conductive and convective heat transfer.
The rate of heat transfer to the individual solder balls usually is not uniform throughout the ball grid array. In particular, the heated air primarily contacts the outermost surfaces the connector, and the outermost solder balls in the ball-grid array, i.e., the solder balls located proximate to the outer perimeter of the ball-grid array. The outermost solder balls therefore tend to receive a higher amount of thermal energy than the innermost, i.e., centrally-located, solder balls.
The need to transfer sufficient thermal energy to the innermost portion of the ball-grid array to melt the centrally-located solder balls can be addressed by slowing the speed of the connector and the substrate through the convection reflow oven, i.e., by increasing the dwell time of the connector and the substrate in the oven. This approach can lower the yield of the oven, i.e., number of connector and substrate pairs that can be processed in the oven per unit time.
Alternatively, the temperature of the heated air within the convection reflow oven can be increased. This approach, however, can result in unintended connector, substrate, or component damage.
Moreover, BGA and other types of connectors typically operate at temperatures above ambient. Temperature changes can cause the connector and its mounting substrate to deflect, i.e., to expand or contract. The amount of deflection of a component as a function of temperature change often is expressed as the coefficient of thermal expansion (CTE) for the component. The amount of deflection experienced by the connector and substrate in response to a given temperature change usually differs. In other words, the CTEs of the connector and the substrate are usually different.
Differences between the amount of thermally-induced deflection of the connector and the substrate can induce stresses on the solder connections between the two components. These stresses, repeated over multiple heating and cooling cycles (referred to as “thermal cycling”) can weaken the solder connections. Weakening of a solder connection can affect the integrity of the signal transmission through the solder connection, and in extreme cases can result in separation of the solder connection from the connector or the substrate.